Seven checks you can do safely
- Thermostat: set to Cool, fan on Auto, and the setpoint at least 3 degrees below room temperature. Replace the batteries if the screen is dim.
- Breakers: check the AC and furnace breakers in the panel, and the disconnect switch beside the outdoor unit.
- Filter: a clogged filter starves the coil of airflow and can freeze it. Replace it.
- Outdoor unit: is the fan spinning? Clear leaves, weeds and anything within two feet.
- Ice: look at the copper line at the outdoor unit. If it is frosted, switch the thermostat to Fan only for two to three hours to thaw.
- Drain switch: many systems have a float switch that shuts off cooling when the condensate drain backs up. Look for water in the pan under the indoor unit.
- Vents: make sure supply registers are open and returns are not blocked by furniture.
When to stop and call
If the outdoor unit hums but the fan does not spin, or it clicks and will not start, the most likely cause is a failed capacitor or contactor. Those carry high voltage even with the system off and should be replaced by a technician. A frozen coil that returns after thawing usually points to low refrigerant or an airflow problem. Both are routine AC repairs.
Why it happens in heat waves
Capacitors weaken over time and heat accelerates it. The first 95-degree afternoon in Warm Springs or Milpitas is often when a weak one finally fails. A spring tune-up tests capacitors against their rating so they get replaced before they leave you without cooling.